Visible crew/equipment: After leaving her husband at train station, Harriet takes a cab to meet a man at a cabin. After a few mins, she takes the man's car to go home. As she drives away, there's a reflection of a man, in suit with glasses (possibly the director) standing offscreen. (00:05:20)
Other mistake: George Tabor parts his hair on his right side, however when Perry cross examines Mr. Tabor his hair is parted on his left and then reverts back to his right side. The two dots on his tie are one low and one high, they also switch. The negative must have been flipped.
Revealing mistake: Secretary Della Street answers the telephone in Perry's office and speaks briefly with the owner of an art gallery. She presses the "hold" button, then hands the receiver to Perry and says, "It's Mr. Clune." Perry and Mr. Clune have a lengthy phone conversation, even though Della never released the "hold" button.
Other mistake: Park Milgrave finds a small chest full of realistic counterfeit money in Bruce Strickland's apartment. (Counterfeit money was not used for this scene. The chest actually contained three stacks of dollar-sized paper with a real bill on the top of each one.) Milgrave dumps the money into a metal urn and lights it on fire but the money he dumps out is play money, which does not look even remotely like real money.
Other mistake: In an abandoned house, a 14-year-old boy finds a satchel filled with money taken in a holdup at a finance company. He uses $50 to buy a robe for his grandfather, then hides the rest of the money in his grandfather's house. During the trial of the grandfather, who is accused of being one of the holdup men, the police lieutenant says, "We found all the money except for $12,000. We conducted a second search of the home and found the rest of it." He should have noted that $50 was missing.
Other mistake: Perry, Paul and Della attend a Wildcats football game at the Los Angeles Memorial Coliseum. Stock footage shows a packed stadium. After the game ends, a close-up shows the three exiting the stadium along with several other people. The next scene takes place outside the entrance to the Coliseum. In a long shot, only two cars and six people are visible. In 1965, the Coliseum's seating capacity was 101,574. Where did everyone go?
Plot hole: Claire Allison spends the night in an upstairs bedroom in the Jennings' home. In the middle of the night, she opens a walk-in closet to get another blanket and finds a small printing press. Later in the day, Claire returns to the home, accompanied by Perry Mason, who tells Mr. Jennings that Claire has received threatening letters from Mrs. Jennings' former husband, who still has a key to the Jennings' home, and the printing press may have been used to address the envelopes. Mr. Jennings says, "We'd better investigate" and immediately goes upstairs and opens the closet, even though neither Claire nor Perry Mason said where Claire saw the printing press.